Magnetometer circuit



June 21, 1960 F. B. coKER MAGNETOMETER CIRCUIT Filed Nov. l2. 1953 SNN @/QLTM@ (n 1 1 4 N IVAA/K. COKEQ, JNVENTOR.

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nited States Patent 2,942,179 MAGNETOMETER CIRCUIT AFrank B. .Coker, "MontereyParkyCali, assignor, by

, mesneassignments, toUnited Geophysical Corporation, lPasadena, Calif., a` corporation of- California yWhen the flight is over water, lthe anomaly` sometimes .indicates thepresence of submarines or vother `metallic sea-going vessels. 'In `other cases, whether theilight is .over-,the sea or overl Ian"d,such anomalies aid in`locating formations that are "likely to contain valuable mineral `deposits such as `oil or metallic ores. Such magnetometers may also be carried by automobilesior otherwise by land over the earths surface in order to locate such` :anomalies indicative of thefpre-sence of mineral deposits. The strength of the earths `magnetic iield is Vof the order of DIS ,"Gauss. Signifcant differences 4in `the .strength of the magnetic heldin, suchananomalous area tare often of the order ofless than 0l% of the earths e1d.and are .so small compared to the total strength of `the eaiths magnetic field that 4the measurements of theiield strengthfare usually expressed in 7, where Because of the fact that such small differences inthe strength of the magnetic field may be Asfigniticanhit is important that 'the magnetometer employed for :its measurement be `very sensitive and-also very stable. Unless it is sensitive, small, signiiicant anomaliescannotl be detected at all. "Unless it is stable, the measurements may'be masked by 'long term uctuations .sometimes fcalled drift or` by random rapid variations, sometimes "called noise In order to achieve `the `desired degree of accuracy systems have been developedin `vifhic'ha magnetometer .eletment 4detec'tsrthe strength of the magnetic 'eld at-a particular point at which it is desired to make ameasure- :ment and the output of the element` is. employed to op- ,'erate various electrical devices including amplifiers, motors, rectifers and sources of current Vto .generate Ta compensating magnetic 'eld whichtacts on theclement in opposition to ,theambient magnetic ield. .Systems `of `this typeare shown by wayof eXampleinfFrowe, .Patent No. 2,584,571 and Brewer, Patent ANo. 2,606,229. `In `:the Frowesystem, the .magnetometer element remployed 4for detectirggA themagnetic `field` is.` infthej form of a -Wind- Ling which vrotates 'about `an .axis Jperpendicular lto Athe -zcompouent of the magnetimeld "being detected. In this ,ssysternhthe compensatingpmagnedtic `ield is regulated `sulicient to balance the residue.

2,942,179 Patented June 21, 1960 automaticallyby means of a servo-mechanism. `Inathe Brewer system, the magnetometer elements-lareioflthe In the latter system, theadjustment of the compensating magnetic iield is `producedautomatically by means of ainegativefeed-baclt circuit. `lor convenience the term feed-back loop is employed' here- Einafter to` refer tothe series of elements-thatproducethe automatic compensation regardless of Whethertheicom- `In both the'Frowe andtheBrewer et al. Ysystenflsfas Well as other systems in which .la-compensating` magnetic viield is generated automatically, an auxiliarysource of 4current'is `employed to bucltoutvalmost"all` o'f ther-mag- 15 1 netic field being measuredand-th'e controlwsystem isii'equired to produce an additional-compensatingfcurrent In suchsystemstlie auxiliary current source is subject -to uctuations and dritt, some of which -may result from `temperature 20- changes, 4some of which may occur -spontaneouslyyand some of which may be `clue to aging causedf byithe drain of current drawn from the source. Such changes inthe auxiliary current source cause corresponding changeslin the unbalanced residue of thelmagnetic'eld which is being measured.

Consequently, in lorder to obtain accurate measurements of the tield strength `with such prior systems, it isneeessary to accurately measure theY current being suppliedfrom'the auxiliary sources as well asthe-cuirent `being controlledby'the feed-back loop and to corn- Vbinethe readings of the twol currents. The requirement of reading both currents accurately maybe satisfied-by employing itwocurrent indicating ,devices-connected to the two sources. This requirement may `alsobe satis- `tied 'by' providing an-Sarrangement for alternately measur- .inglthe` currents supplied bythetwosour'ces. The first methodis unduly expensive and impractical. LThefsecondv method is subject to other diiiiculties. Furthermore,

when it is desiredtoproduce aI continuous recording-"f `the variationsinlthei magnetic Vtiel'dstrength 4along a line ofLexploration, it` is highly fdesirablet that the record ma'de represent the variations in the field --strength lfromfpoint to '.point without requiring any correction for uctuations orshiftsin `current produced inan auxiliary lcircuit-"or otherwise. Also, in order that accurate readingsmay be obtained as 'the magnetometer t is carrie-d along aline ofexploration, itishighlydesirable that the part fthe system that` controls the compensatingmagnetc `ticl'drespondsubstantiallyinstantaneously.

'Whileit is desirablethat the magnetometer employed .bercapableof measuringand 'indicating accurately large differences in magnetic field strengths between various points,such as `diiierences of about 10,000l y or more,"`;it is .also desirable that the magnetometerfindicate accuratelydiierences in strength of-the order `of l Py. Alsojin order.A that accurate readings mayibe obtained `as the magnetometer travels along a line of explorationjitfisfhighly `desirable thatthepart of the `system that controls the Acompensating magnetic :field `respond instantaneously.

substantially ,An object vof this' inventionsis'to provide la-magnetometer of the type `describediin which `the compensating .field isproducedrby varyng'acurrent suppliedifromfa single current sourcein response tothe net magntictield `to which themagnetometer elementfisl exposed.

...system in which the signal produced in response to the magnetic eld is amplified without danger of introducing spurious signals and in which the amplified signal is then employed to vary the current supplied from a single current source to produce the compensating magnetic eld.

Another object of the invention is to provide a system for setting the strength of the compensating magnetic field initially at a value which will permit the feed-back system to operate efiiciently to balance fluctuations in the residual magnetic field without impairing the accuracy of the observations.

Another object of the invention is to provide a magnetometer which employs a neutralizing-current source whichV need not be extremely stable in order to achieve a high degree of accuracy.

t Still another object of the invention is to provide a system in which a large portion of the compensating current may be neutralized and accurate measurements of the residue may be made without introducing any substantial errors that would be indicated or recorded as spurious parts of the magnetic field.

Y A still further object of the invention is to provide .a magnetometer in which the sensitivity and calibration may be easily checked.

A still further object of the invention is to provide a magnetometer having an output which can be recorded as the magnetometer travels over the surface of the earth and which will indicate accurately the differences in strength of a component of the magnetic field at different points along the line of travel.

A still further object of the invention is to provide a magnetometer in which the sensitivity and the zero setting of the indicator may bev adjusted independently of each other and without altering the current that produces the compensating magnetic field.

, A still further object of the invention is to provide a magnetometer with a recorder of the self-balancing potentiometerrtype which employs as a source of reference voltage the same source which supplied current to produce the compensating magnetic field.

The foregoing and other objects of the invention together with various advantages thereof will be set forth in the following description of one embodiment of the invention which is illustrated schematically in the single figure of the drawing.

l While this invention is applicable to magnetometers of other types, it is illustrated and described herein with particular reference to its application to flux-gate magnetometers that employ unbalanced bridge circuits. Such a system is shown in the single figure where current from an oscillator is impressed upon an unbalanced bridge circuit 20 employing a pair of flux-gate magnetometer elements 22 in one branch thereof. The twoV magnetometer elements 22 comprise separate windings wound on a common magnetic core 24 that is in the form of a thin lamination or strip of ferromagnetic material which has Y high permeability and a B-H curve that is characterized by a sharp knee at saturation. A resistor 25 connected across one of the windings 22 serves to unbalance the i bridge electrically.

4In order to measure the strength of a component of a Y magnetic field such as the earths magnetic field along a partlcular direction, the axis of the core 24 is aligned" Vthe strength of the entire magnetic field of the earth is to be measured at a point, the core is mounted upon a gimbal mechanism which is provided with a pair of auxiliary :magnetometer elements that are arranged along axes per- Vpendicular to each other in a plane perpendicular to the axis to the core 24 and suitable means O f e type WGH' plitudes of the pulses is substantially constant even if substantial changes occur in the ambient magnetic eld to which the magnetometer is exposed. However, the arithmetic difference in the `amplitudes of the pulses is proportional to the strength of the component of the magnetic eld which lies along the axes of the elements.

According to the present invention, the output of the bridge-circuit 20 is employed to control the current supplied from a single current source 30 to produce a magnetic field which has a component at the core 24 that is substantially equal and opposite to the strength Vof the component of the ambient magnetic tield being measured. The control is effected substantially instantaneously to avoid spurious effects that might be introduced other wise. The various means by which this control is effected to produce highly accurate and reproducible results are considered in detail hereinafter.

The current source 30 is connected in series with a field lbalance resistor R1, a precision resistor R2, and a variable resistor R3, in the form of a triode and this series of elements is connected across the output of the bridge circuit 20. With this arrangement, current from the current source 30 flows in the current loop L2 through the precision resistor R2, variable resistor R3, and the two windings 22 of the respective magnetometer elements, and thence through the corresponding halves of the secondary winding 12 of transformer 11 and through the field balance resistor R1. When the two magnetometer elements are formed by separate windings onra common core 24 as in the case illustrated here, the two windings 22 are so wound that the magnetomotive forces applied to the core 24 by the flow of-current through those windings from the source 30 are in the same direction in the core. In practice, the connections are so arranged that the compensating magnetic field produced in the core 24 by the current flowing in the windings 22 is in a direction opposite to that of the component of the ambient magnetic field to which the core 24 is exposed. With this arrangement the magnetometer element responds to the component of the net or resultant magnetic eld that is parallel to the core 24.

In order to produce a compensating magnetic field which balances the ambient magnetic field, the output of the bridge circuit 20 is passed through an amplifier 40 and thence to a full-wave rectifier 50. The amplifier 40 includes a blocking capacitor 42 at its input in order that all of the single compensating current shall pass through the magnetometer windings 22. A saw-tooth wave appearing at the output of the rectifier is then passed through a combination amplifier and band pass ilter 70 and a phase detector 80, all as explained in more detail and claimed in oo pending patent application Serial No. 391,576, filed by Frank B'. Coker, November 12, 1953. By employing a class A power amplifier as the amplifier 40, a high degree of accuracy is attained as explained in said application. In the system described in that application, the charging circuit of the rectifier 50 has a time constant which is about equal to or greater than the period of the waves supplied by the oscillator 10 and the amplifier-filter 70 is adapted to pass only the harmonic component of the saw-tooth wave which is of the same frequency as thel current supplied .by the oscillator 10. With such an arrangement, an alternating current appears at the output Vof the combination amplier and filter 70 of the same lfrequency as the Waves applied t9 the bridge irrcuit 20ans of an amplitude proportionalffto theustrength of-Ithefnet componentfdf "111e :magnetic :field tto which the core 'i1-24 A"-is exposed. Furthermore, the phase of thealternating current at the `output of.` the combination amplifier and filter 70 reverses ywhen the directiontor sense of the magnetciield comyponentrlying along the axis of the element` 24 is reversed. The magnitude of the output of the `amplifier-70 ymay `be measured` by means ofsa meter.4 85 for purposes to beexfplained in detail hereinafter.

The output of the amplifier "70 ispassedthroughtthe Aphasezdetectorti and D.C. amplifier 90 to` produce a D.C.` voltage for varying thebiasofthetriode R3, the fbias being .so varied .as to alter the `current flowing from the 'current :source 30 through. the windings 22 in such fatway .as to automatically compensateforany changes in the-strength of theambient `magnetic field. Thus, with 'thiswarrangement if the strength of thei ambient magnetic .field towhich the core 24 is exposed increases, the output `of;thenanflplifier 70 increases -in `a corresponding manner @causing a corresponding increase in the VD.C. `Voltage supplied by the 1D.C. amplifier 90 to the `resistor R3.

-Withf the output of the DC. amplifier 90 so connectedas tto vreduce the bias on the triode R3, the resistance of' the `triode decreases land the current flowing `through the fwindings22 increasesin order to increase Ithe strength of' the compensating magneticfield =by an amount which is substantially equal to the increase in `the yambient magnetic field. Likewise, with -this arrangement, la `decrease in the strength of the yambient magnetic field `causesthe value of the current supplied to the windings 22 to be adjusted automatically by Yan amount to produce `acorresponding decrease in the strength of the compen- -sating magnetic field. The appropriate `connection of the phase detector 80 and the D C.` amplifier l90 to the trio'de 'R3 is indicated by -land signs in thediagramfshowing that an increase in ambient magneticrfield strengthV adds a positive voltage to the bias `existing on ythetriode R3 Athereby decreasing the bias and a decrease in the ambient magnetic 'field `adds a negative voltage to the bias thereby `increasing the bias. LThe action of` the feed-back loop L1 Athat produces the compensating is very rapidlbeing sub- -stantially instantaneous.

As morefully explained in said copending application, the employment of a class A power amplifier at the `output of tbe unbalanced bridge circuit has the advan- `tage over other types ofamplifiers in that positive and negative pulses appearing at the output of the bridge -circuit are amplified in equal, almostconstant `and unvarying proportions so that the difference in amplitudes of such pulses is accurately indicated -at' the output oft the amplifier '70 without danger of introducing spurious readings that might otherwise `be introduced such -asby variations in characteristics of the amplifier tubes employed in thearnplifier 40. As explained in that application, the Areliability and accuracy `of ythe results indicated `at the .output of the amplifier 7f3 isless dependent upon the characteristics of thetubes employed in the amplifier 40 for resultant magnetic fields of small amplitude thanfor resultant magnetic fields of large amplitude. It is thus seen that by employing a class A power amplifier in the feedlback loop L1 the magnitude of the compensating magnetic field canbe varied automatically to balancethe magnitude of the ambient magnetic field without danger of introducing spurious effects due to changes in the characteristics of the amplifier dit. Furthermore, the em pioyment of such a feed-back system causes the corn- `pensating magnetic field to have a valuethat is substantially independent `of changes in characteristics o'f the 'maintained at a valuefwhichnearly exactly balances out ythe input of the chopper is `reduced `to zero. point the fraction of thereferencevoltage applied to the regatta-r7.9

the ambientrnagnetic fieldset: thermagnet-ometer element 24.

The potential producedv across thei precision resistor R2 by the compensating current'flowing therethrough is` thus `proportional Ito the strength of the compensating magnetic field. In order to measure the Value of the current producing the compensating magnetic field, fa self-balancing recording potentiometer is employed.

The potentiometer 100is of `a conventional type which is adapted to compare two voltages and to displace its recording element .102 from a zero position to a recording position by a. distance `corresponding to the difference between the two voltages. In practice, recording paper .-104 is moved continuously by a motor past the 4recordingselement 102 as the magnetometer `is carriedalong a line of exploration, in order to continuously record variations in the strength ofthe magnetic field ofthe ear-th. from pointwto point along-that line.

The self-balancing:potentiometer 100' shownfemploys a..chopper.106,.an.A.C. amplifier'IOS, and'a balancing motor 110. The balancing motor 110 is connected-to the `recording element 102 and to a` sliding contact105 of a potentiometerresistor R5, across which a constant 'reference voltage is applied. Al fraction ofthe reference voltage, as determined by the positionuof the contact 105,

-is applied to input terminallzr Vand `a voltage'to be lmeasured isapplied to another input terminal 114. In

practicethe difference in voltage appearingat the `terminalsof 112 and 114 is converted by the chopper 106 into an alternating square-wave voltage which ispasse'd through ytheAC. ampliiier 108 to Vdrive thetmotor 110 in such a directionas to move thecontact to .as position on the resistor R5 `at which` the voltage. applied to At this input terminal 112 equalsthe voltage to Vbe measured. Inasmuch as the recording `element '102.is connected directly to the sliding contact 410Sthe position of the recording element 102 accordingly indicates the `value of the-voltage appearing at the input terminal 114. With such-an arrangement if theiresistor R5 is linearfthat` is if its resistance per unit of its length or displacement of the slider, is uniform, changes rinvoltage atv terminal 114 -produce proportional displacement of thel recordingv eletment 102 on the recordingrpaper 104.

The voltage impressed upon the input 1I141of the selfbalancing potentiometer1100 is derived from theprecision resistor.R2. More particularly, `the ungrounded end of theresisto-r R2 is connected in series with batteries B1 :and B2 to input terminal 114 of the potentiometer "100.

The connection to the battery B2 is madeI through azeroshift resistor or potentiometer R4 so that an adjustable portion of the voltage lfrom'battery B2 and all of the voltage of the `battery B1 are in series with the potential Vappearing across the precision resistor R2. The total voltage thus introducedin `series is slightly'less than `the potential that appears across the resistor R2.

`Theftwo batteries B1 and B2 are of a very stable type which are free from random voltage fluctuations an'd which are free from gradual voltage drift, when littleor vno `current is being drawn from them. `vBatteries of 4a type which .have proved eminently successful forthis Apurpose are mercury cells manufactured by P. R. `Mallory & Company, of North Tarryton, New York, such mercury cells being manufactured and sold under the designationiRM 1 and'RM 12. The battery B1 produces a much greater voltage than^battery`B2land in fact" battery kB2 and `potentiometer R4 serve only toprovide a slight .adjustment in the total voltage difference ybetween the yterminal `114 and the arm of potentiometer R4 Thus,-`for "example, batteryBl may consist of 15`RM-l cells which `are 1rated.at.1.345 volts each While'battery B2 mayconsist of only' 3 RM-12 cells which` areratediat 1.2345 -volts Y resistor R5.

".7 the drain on the cells of battery B2 is less than about 0.2 ma., and with no substantial drain on the cells of battery B1, i-t 'is found that no substantial fluctuations or drift of voltage occurs in these cells which would interfere with the accurate measurement of changes in the current flowing through the precision resistor R2 to an accuracy of about l part in 50,000 parts.

The motor 110 and chopper 106 are driven by power derived from the oscillator 10.

A low pass filter F having a cutoff frequency which is very low compared with the frequency of the oscillator 10 is employed in order to prevent any appreciable signal at that -frequency lfrom being applied to the input terminal 114 of potentiometer 100, Thus when the Vfrequency of the oscillator is 400 clp.s., the cutoff frequency is about 5 c.p.vs. The use of such `a filter F is very advantageous when the chopper is driven at the same frequency as that applied to the bridge circuit. In such a case the filter prevents any systematic errors that might be introduced by the synchronous rectifying action that the chopper 106 would apply to signals of 400 c.p.s. reaching the potentiometer 100 from the precision resistor R2. A calibration circuit 120 is connected in the lead between the ungrounded end ofthe precision resistor R2 and the input to the potentiometer 100.

The other Voltage that is to be compared in the potentiometer 100 appears across the variable potentiometer To provide the desired reference voltage, the potentiometer R5 is connected in series with a sensitivity-control resistor R6 or R5" across the output of the current source 30. By manipulation of a sensitivity control switch S1, one or the other of the resistor R6 or R5" may be selectively connected in the circuit to change the value of the current flowing through the potentiometer R5 by `any desired ratio. Any variations in voltage appearing across the output of the current source 30 are reproduced in proportional amount across the p-otentiometer R5. However, since the value of the voltage impressed yfrom the precision resistor R2 on the self-balancing potentiometer 100 is only a small fraction of the total voltage across'resistor R2, uctuations in the voltage appearing across the potentiometer 105 a-re of little significance as explained more -fully hereinafter.

Y The voltage appearing between the sliding contact 105 of the potentiometer R5 and ground is matched in the self-balancing potentiometer 100 against the fraction of Vthe voltage applied thereto from the precision resistor R2 and the batteries B1 and B2 as explained above. Under these conditions the sum of the voltages in the potentiometer loop L2, including the resistor R2 the batteries B1, and'B2, and the included portion of the potentiometer R5, is zero.

The calibration circuit 120 is of a simple type which is adapted to inject a voltage of standard value in series between the input terminal 114 and the ungrounded end of the precision resistor R2. When such a voltage is injected in the lead in question, it in effect introduces a virtual, or apparent, change in the compensating magnetic field and the self-balancing potentiometer 100 displaces its recording element 102 by an amount corresponding to the magnitude of the virtual change in the strength of the magnetic field. The Value of the voltage so injected for this purpose is chosen to represent a predetermined change in magnetic field strength of, say, 100 7. Accordingly, whenever, it is desired to check the calibration of the instrument, it is merely necessary to inject the calibrating voltage temporarily.

This calibration circuit is very effective because the magnetometer element 24 has substantially constant D.C. current sensitivity which might be expressed in units of ,'y/ma. Vfor example. In addition, the null type of operation provided by the feed-back of the compensating curcurate measure of the ambient magnetic' eld being measured. This condition is'not upset by variations in the magnetometer circuit such as changes in the amplitude of the energizing current supplied by oscillator 10, or by changes in the sensitivity of the bridge circuit 20 to changes in magnetic` field strength. For this reason the introduction of a standard change of potential into the potentiometer input causes the pointer 102 to move to a new position which is a direct indication of the effect which would be produced by a corresponding change in the ambient magnetic field. The value of the potential change introduced to provide the effect of a fixed change in magnetic field depends only upon the currentv sensitivity of the element 24 and the resistance of precision resistor R2, both of which are very constant values. In practice, then, the introduction of this standard change in potential provides a direct calibration checking means.

If it is yfound that the injection of the Calibrating voltage does not produce the desired displacement of the recording element 102, the potential across the output of the current source 30 is adjusted to produce the voltage across the potentiometer R5 required to produce that displacement. To permit such adjustment without disturbing the remainder of the circuit, the current source 30 is of a type which includes a regulated voltage supply including a series tube T1 and a shunt tube T2 for automatically regulating the value of the voltage appearing across the output, irrespective of changes in Voltage occurring in the battery S or other source of D.C. voltage at its input and irrespective of variations in the load circuit supplied 'by the source. The magnitude of the voltage appearing across the output is varied by adjusting the position of a sliding contact 32 of a potentialdividing resistor 34 shunted across the output of the current source 30. The adjustment of the sliding contact 32 varies the proportion of the output voltage that is impressed on the control grid of the shunt tube T2 and hence the output voltage itself because of the change in feed-back ratio. Inasmuch as the impedance of the current source 30, as measured across its output, is very low compared to the resistance values of the various circuit elements connected thereto externally, the effective value of this resistance is not changed substantially by movement of the sliding contact, even though the voltage appearing at the output may be changed considerably.

In practice the feed-back loop L1 is capable of compensating `for variations in magnetic field strength over -only a limited range of magnetic field strength. This is due partly to the limitations in the range of operation of the unbalanced bridge circuit 20 and partly to the fact that the elements of the feed-back loop L1, including the class A amplifier 40, the variable resistor R3, or intervening elements, become insensitive when changesin magnetic eld beyond a predetermined value are applied to the magnetometer 24. For this reason the field balance resistor R1 is employed to set the magnitude of the compensating field at a value about equal to the average value of the ambient field that is likely to be encountered in the course of the investigation. For example, the amplifier feedback loop L1 may be adapted to operate very eectively over a range of about $5,000 v. In such a case, if variations in magnetic field strength of less than 10,000 y are to be measured and the average value of the total magnetic field strength that is likely to be encountered is 50,000 ry, the value of the resistor R1 is set at a point such that the compensating field produced at the core 24 is about 50,000 fy. The actual value lof the average compensating field is computed from the value of the potential of the battery B1 and the setting of the resistor R4, the instrument having previously been calibrated to make such computation possible.

When the instrument is so set, the position of the pointer tliti.fcorrespondingthereto is Va zero or reference :position and' displacements ofi the' pointer `therefrom `indicatedeviations in the strength of the Am agnetioeld: at `varionspoints from the valve of the ambientiield for whichthe balancing field was originally set. In practice, the value of ther-esistor R4 is so set that'the reference levelfor the pointer isfintermediatethe width ofthe recording strip. -With `.such an arrangement both positive and negative deviations are readily indicated within the limits i5'000 fy.

:When-the system is first placed in operation, and also wir-heul thevolt-age of the current source` 30 is changed to adjust: the Ica-libration, it. is desirable to manuallyadjust thevalue'ofthe D.C. compensating current flowing in the bridge circuit 20. to a value which, without the correcting effect of the feed-back circuits, willsubstantially compensate tthefmagnetic field in whichthe element 24 exists. `When '.suchadjustment has :been made, Vthe instrument thereafteriscapable of themaxirnum rangeof operation in -both the positive and negativeisenses, i.e., increases or 'decreases'in the magneticteld. To make this adjustment, 'ftheswitch 94 is first depressed thereby shouting the output of fthe-D C. Aamplifier 90 and rendering the feed-back action ineffective. Then, while no feed-back action is Voccurring,.the.f`1eld balance resistor R1 isY adjusted until -thebalance indicating meter SSxreadssubstautially zero. .Thetswitch 94 is then opened,irendering the feed-back -loop operative again. After Vthis adjustment has been made, the feed-back action of the magnetometer feedtbackftloopfwill thereaftermaintain the D.C. compensating feld* at themagnetorneter bridge 20 at nearly exactly the value necessary to compensate the ambientmagnetic field existing at the element 24 and the range of operation available will bea maximum in either the positive or negative direction about the value ofV magnetic field` for which lthertadjustment was made.

It is to be noted that the operation ofthe feed-back looplLI toproduce a single compensating current, oers advantages `not available in other systems. One advan- ,tagefof employing such a single current controlled en- .tirelyz'bya feed-back system. for compensation, lies in the fact that variations in the values `of various elements in the system have little or no effect on its value. .As a result, the measurement of this current provides a reliable and accurate measurement of the strength of the am- :bient eld. Variations in lthe values of eld balance resistor R1; in the characteristics yof series tube R3, in the gain icharaoteristics of the D.C. amplifier"90, in the characteristics of filtering and amplifying circuits 70, .or the phase idetectingcircuit 80, as well as changes inthe D.C. resistance of any portion of the compensating current loop :comprising thezcurrent source 30, magnetometer bridge .120g-resistor Rband the resistor R2 have substantially no .'effeotupon the valueof the compensating current. Because the feedbacklaction operates substantially instantaneouslywlien any such variations occur, it automatitcally'becomes ineffective orunnoticeable lalmost irrimediately and the compensating current isreestablished quickly -at substantially the same value` it had` prior tothe oc- "c'urrence ofthe variation.

The system of this invention possesses-:great Aadvan-`- tagesover` one that employs more ,than one source of current to produce the compensating field, for example, one femployingftwo' sources, one of which compensatestthe 'majorportion of the ambientield at the magnetometer felement, while the other automatically provides additionalzcompensation to reduce 'theresidual magnetic field at the magnetometer elementto substantially zero. In such a system, variations in the value of thecurrent that compensates the major portion of the. magnetic field, cause corresponding variations tooccur in lthe other, automati- .cally controlled, current. The latter variations are re- Acorde'das :spurious changes inthe ambient magnetic field =being measured, leven though no rea-l change corresponding thereto has occurred. In contr-ast with suchfatwollsourcefsystem, the present invention `creates a single com- .F10 pensatingfieldfroma single; compensatingfcurrentwhich isgmaintained:.atl substantially .the .exact value necessary tofenti-rely balance out the magnetic field-at the detector elementl24.

TThefrecording system employed in this inventionpermitsfull exploitation of the foregoing advantages of the singlecompensating current, especially where, as ingeophysical prospecting, .the variations of the magnetic eld .are onlya smallpercentage of the total eld being measured. In the present system high accuracy is obtained byfbalancing `out `nearly all of the potential across precision res-istor.=R2 by the stable batteries Bland B2. By wayof illustration, if the total magnetic field being measured is about 50,000 'y, and the corresponding-voltage that appears across the precision resistor R2 is 10 volts, the sum Aofthe potentials producedbybattery` B11 and the. fractionuofbattery B2 selected by'. the adjustment of the potentiometer :Rpmayvbe set at about 9.5` volts. Thusthe total potenti-alappearing between terminal 114` and ground `wouldfbe only 0.5vvolt and variations in this potential mightbef expected to range fromzeroto 1 volt, thus requiring that theatotal voltage across potentiometer vR5 be 1 volt. In. sucha case, it is evident that the total range of -operation of the recording potentiometer l0() is JAG of 50,000 'y, or 500() fy.

It-is-irnportantto-note .thatthe balancing motor 1110 automatically movesthe contact of potentiometer R5 to theposition for-Which no dilerence in potential exists betweenthe terminals 112 and 114 so that the sum total of the potentials of all of the sources in the potentiometer Aloop otherAt-hanthat across R2 is exactly equal to the `potential across resistor R2. When this condition exists,

`no currentfflows in resistor R2 other than that which is being measuredglandau of this current flows through `.the

bridge circuiti-20. LByf'employing a potentiometer 100 in whichfsubstantiallyuno part of the .compensating current is shunted theretotf-rom the` bridge circuit 20,l accurate `measurements.:maybe made of -thermagnitude ofthe comp ensating. current.

As explained hereinabove, the single compensating current is .passed through .a high precision resistor R2 thereby producingtauvoltage thereacross which is always proportional to the value of that current to -withinY a very high accuracy. The overall accuracy and stability of the potentiometerrecording system i100` vthus depends primarily upon the stability of the series batteries B1 and B2 `andthe `valuelof the current owing through potentiorneterfR5q As'previously mentioned, batteries B1 and B2 tarepfaa'type `providing stable output potentials under conditions under which small .currents pass therethrough 'and .thus` theposition of the pointerlltlz` on .the recording paper.. 105:.provides' `a direct and highly accurate indication of any/changes fin the magnitude of .'thelcurrent flowing in :resistor R2. iln'prac'tice, the potentialsy .produced by batteries B1.andfB2 :are'stable to within about l part in 450,000 parts during substantial periods of operation of `the .order offthoursso that errors from .this source are lessi thanl Jy. lover .such periods.

'As previously explainedgvariations in the output voltage of the current.- sourcentl .are ofi little or no conse- Aquence intheioperation off the'feed-back loop L1, since -anyivariationin thecompensating current are almost in- 1st`antaneouslycompensated `for.` .by the feed-back. action of this loop. '-However, 4variations 1in the'source of potential T10-cause -"some variation in the indications of the recording systemassevidenced by the movement of the pointer ltizbbecause of the effect that these variations have on the current flowing through resistor R5. However, sincezthe. total voltage across resistor R5 is only a small `fraction,such-asabout l/o, of the total potential existing `across-'precision resistorR2, iteis apparent that,

for a givensaccuracy, the variations in the voltage of in this case about %0, of the voltage appearing' across resistor R2. For example, in order to maintain a recording accuracy of t5 y'out of ay total of- 50,000 y, variations in the output voltage of the source 30 may be as great at .01%. But if the stable batteries B1 and B2 were not employed, variations exceeding .001% could not be tolerated Vin the output voltage of source 30. Thus the bucking-out or balancing'of substantially `all of the voltage to be measured by`means of a highly stable set of batteries, and the balancing of the remaining fraction of the total voltage to be measured against that produced by a current source, permits -the use of a relatively unstable lregulated current source .to obtain a relatively high degree of accuracy.

Another feature of the recording loop which provides for convenienty operation thereof is the zero shift adjustment provided by ythe combination of the battery B2 and potentiometer R4. It will be seen that when the potentiometer recording loop is in a balanced condition and the balancing motor lill has ceased to operate, thereafter an adjustment -of the arm of potentiometer R4 causes an unbalance in the potentials in this loop and hence causes the A C. amplifier liiS to operate the balancing motor 110 in such a direction that the contact 105 of the potentiometer R5 will be moved :to a new position in which the total voltage in the potentiometer loop is zero again. Thus .the potentiometer R4 may be adjusted to set the recording element 102 at any -desired position on the scale that may be required for convenient operation thereof.

It will be apparent, of course, that in order to partake of the advantages of a primary object of the present invention, which is the creation of a single D.C. current, the magnitude of which is always proportional to the magnitude of the magnetic iield being measured and the measurement and recording of the value of said current -by means of a self-balancing recording potentiometer, it

is not necessary to provide a single source of power to act in the dual role of supplying both a portion of the reference voltage yfor the recording potentiometer and supplying current to' create the compensating magnetic field at the magnetometer element. For example, one current source, such as any battery could be employed to supply the compensating current and an additional -set of mercury cells, for example, could supply the curwill be understood of course, that the .triode R3 may be replaced yby a variable resistor of a type having a sliding contact `and that the value of this resistor may be controllcd by the employement of a servo-motor operated by the output of the `ampliiier-iilter 70 to adjust .the position of .the sliding contact automatically to produce a compensating current of .the correct value.

It will also be apparent that the current may be controlled in other Ways than by varying the resistance of a triode R3 in the current loop L2. For example, the current might be controlled by applying a voltage from the output of the D.C. amplifier 90 in series in this loop at the point where the triode R3 is connected.

Though `only `one main embodiment and only a few Vvariations or" the invention have been described in which the various objects are attained, it will be obvious that the invention is not limited to those particular embodiments and variations but is capable of being embodied in many other forms. Various changes which will now suggest themselves to those skilled in the art may be made in the form and details of construction, and arrangement ofvelements without departingY from the principles of 12 the'invention. i It is therefore to be understood that the invention includes all embodiments thereof that come within the scope of the appended claims.

The invention claimed is:

l. In a magnetometer:

A magnetic field detector including a sensitive element responsive to a component of the net magnetic eld to which it is exposed in accordance with the strength of said component;

Winding means continuously controlled by a current passing therethrough for producing a single variable neutralizing magnetic tield in opposition to an ambient magnetic -eld toreduce the strength of the component to which said sensitive element is exposed;

A precision resistor;

A single current source connected in series with said precision resistor, two electron dischargev devices connected in series with said source, said source continuously supplying a single direct current through said precision resistor and through said winding means, the strength of said neutralizing eld being unidirectional and proportional tothe amplitude of said single direct current;

Means including a feed-back loop connected to said winding means and one of said electron dischargedevices for varying the magnitude of said single direct current in accordance with the magnitude of said cornponent;

And means for measuring the voltage produced across said precision resistor to indicate the strength of the ambient magnetic eld, y

2. in a magnetometer:

A magnetic field detector including a sensitive element responsive to a component of the net magnetic field to which it is exposed in accordance with the strength of said component;

Winding means continuously controlled by a current passing therethrough for producing -a single variable neutralizing magnetic `field in opposition toan ambient magnetic lield to reduce the strength of the component to which said sensitive element is exposed;

A precision resistor;

A variable resistance means;

An adjustable eld balance resistor;

A current source connected to supply a single direc current through said precision resistor, said Variable resistance means, said field balance resistor, and said winding means, the strength of said neutralizing eld being unidirectional land proportional to the strength `of said single direct current;

Means for varying the magnitude of said variable resistance means as an inverse function of the magnitude of said component, whereby said single direct current -varies 1n accordance with said component;

And means for measuring the voltage produced across said precision resistor to indicate the strength. ofV the ambient magnetic iield.

3. ln amagnetometer:

A magnetic eld detector including a sensitive element responsive to a component of the net magnetic iield to `which it isV exposed in accordance with the strength of said component; Y

Winding means controlled by a current passing therethrough for producing a single variable neutralizing magnetic field in opposition to an ambient magnetic field to reduce the strength of the component to which said Asensitive element is exposed;

geiagrve" resistance means as aninverse function of "the magnitude ofM fd component, whereby said single current varies in "accordance "with said component, l A self-l3alancing ypotentiometer including a contract adjustably `engaging a potentiometer resistance element and including an indicating pointer movable with said contact to atpoint indicative of a voltage `across an input tCircuit;

Avoltage source;

"Means for connecting said precision resistor and said voltage source across the input circuit `of said potentiometer;

'And means for connecting said ,potentiometer resistance element across the `output of said current source.

14.111 a magnetometer:

A magnetic eld detector including `a--sensitiveelement responsive to a component of the net-magnetic `field to which it is exposed in ,accordance-with'the strength 'of said component;

Winding'meanscontrolledbya current passing therethroughfor producing a singlevariable neutralizingmag netic eldinopposition to an ambient magneticiield'to reduce the strength of'the'component to'which saidfsensitive element is exposed;

Apreci sion resistor;

A 'variable resistance means;

adjustable-field balance resistor;

A current source connected tosupplya single current through said precision resistor, said variable 'resistance mefans,tsaiid'iield balanceresistor, and said windingmeans, the strength of said I neutralizing Afield `beingproportional to the strength. of said 'single current;

'Means ifor varyingthe magnitude of said variable `resistance 'means `as an inverse Afunction of -the magnitude of said component, whereby said 'single current varies `in accordance with said component;

yself-balancing potentiometer yincluding a contact adjus'tablyr engaging a potentiometerresistanceV element and including an indicatinglpointer movable with said contact toa point indicative of a voltage across an input circuit;

"A'voltage source;

Means lfor connecting saidprecision resistor Yand `said voltage source across the input circuit of said -potentiometer;

'And `means 'for connecting said potentiometer Aresistance element across `,the output of said current source.

5. `-Ina magnetometer:

A magnetic eld detectorincluding a `sensitive element responsive to a component ,of the net magnetic `eld to which it sexposedin accordance'with the Astrength of said ,C Ompollent n"iilin'ding means controlled bya current passing there- ,through v'for 'producing a singlevariable neutralizing magneticiield in opposition to an ambient magnetic eld to reduce the strength of the component to which said sensitive element is exposed; A precision resistor;

, Avariable resistance means;

vAn adjustable tield 'balance resistor;

A `current source connected to `supply a single current through -said precision resistor, said -variable resistance frrieans,` said 'field "balance resistor, and vsaid winding fm'eans, Ythe 'strength of said `neutralizing iield being prolp'rtional tothe strength of saidsingle current; Means Lfor varying the magnitude of said variable .resistance means as an inverse Vfunction of `themagnitude folf said component, whereby said single current varies in Aaccordance with said component; A self-balancing potentiometer including a contact `adjustallyengaging a potentiometer resistance element .and including an indicating pointer movable with said `c'ontact to alpoint indicative of a-voltage across an input circuit;

p A voltage source;

""Mearnsforconnecting lsaidprecisionresistor `and said toithestrength 'of1said single current;

voltage :source :acrosslthe input` circuit of saidilpoteli tiometer;

lSensitiv'fity F control resistance means;

And means for connecting `said potentiometer resistance element and said sensitivity controlresistancebmeans across the outputfof-sai'dr current source.

magnetic'elddetector `including a `sensitive element responsivef to a component of the net fmafgneticfield to which 4itis A-exposedin` accordance with the strengthfof said component;

`tlVind-ingfmeans controlled by a` current passin'githerethrough E4for `'producing la single variable `neutralizing magnetic iield in opposition toren-ambient magneticiield to reduce the strength of the component tovvhich said sensitive element is exposed;

A precision resistor;

A variablet resistance means;

fadjusta'ble eldbalance lresistor;

A current source connectedto supply 'a single current through -:said -precision rresistor, said variable .resistance means, said field balance resistor, and said winding means, Vthe strength of said neutralizing iield being p'roportionllto the strength of said single current;

Means for -varying ther-magnitude of said `variable resistance means as aninverse lfunction ofthe magnitude of said component, whereby-said single current :varies in accordance with said component;

A slelfbalancing potentiometer including-a contact 'adjust-bl-yaengag'ing era i potentiometer resistance element and including an indicating pointer movable with said lcontact to a point indicative of a voltageacrosstan-input circuit;

A voltage lsouree;

Means for connecting said precision resistor and said voltage source across ithef-input-circuit of `said potentiometer;

Means ffor l'comtlecting said `potentiometer -'resistance element `across `the"output=o'f\sa`id current'source;

And V-means `for `adjusting fthe .voltage available vat fthe output -of said Vcurrent source.

A7. V:Infaimagnetometert Amagnetic eld detector including -a sensitive-element responsive to a component of the net magnetic -e1d fto which it is exposed in accordance Lwith the-strengthlof said component;

Winding means controlled by afcurrent passing therethrough for producing a single variable Aneutralizing lmagnetic field in opposition to an ambient magnetic -fi'eld to reduce-ltherstrengthof thescomponent to which saidsensitive element `is exposed;

A Aprecision =resistor;

VA lvariable lresistance means;

An adjustable iild ibalance resistor;

A current source connected to supply a single Acurrent through said precision resistor, said `variable resistance means;saideldbalance:resistor, and saidlwinding means, the strength of said neutralizing field Vbeing proportional :Means tfor #varying the imagnitude of saidrvariable fre- `sistancelmeansfas .anxinverse function of the magnitude offsaidicomponent,'whereby said single current variestin accordance TwithV said component;

Ariself-:balancing potentiometer including .a contact :adjustablyengagingfa .potentiometer reistance :element -and :including an indicating pointer movable with Vsaid contact "to `a -tpoint indicative Vof :a voltage Vacross an input circuit;

Avoltage source; VMeans ffor connecting `said precision resistor and said vvoltagesource yacross Ysaid input circuit of said ipotentiometer;

:Sensitivity tcontrol `resistance means;

Means for connecting-:said :potentiometer uresistance I element and said sensitivity control resistance means across the output of said current source;

And means for adjusting the voltage available at the output of said current source'.

8. In a magnetometer:

A magnetic field detector including a sensitive element responsive to a component of the net magnetic eld to which it is exposed in accordance with the strength of said component;

Winding means controlled by a current passing therethrough for producing a single variable neutralizing magnetic iield in opposition to an ambient magnetic eld to reduce the strength of the component to which said sensitive element is exposed;

A precision resistor;

A variable resistance means;

An adjustable eld balance resistor;

A current source connected to supply a single current through said precision resistor, said variable resistance means, saidiield balance resistor, and said winding means, the strength of said neutralizing field being proportional to the strength of said single current;

Means for varying the magnitude of said variable resistance means as an inverse function of the magnitude of said component, whereby said single current varies in accordance with said component;

A self-balancing potentiometer including a contact adjustably engaging a potentiometer resistance element and including an indicating pointer movable with said Contact to a point indicative of a voltage across an input circuit;

A voltage source;

Means for connecting said precision resistor and said voltage source across the input circuit of said potentiometer;

Sensitivity control resistance means;

Means for connecting said potentiometer resistance element and said sensitivity control resistance means across the output of said current source;

Means for adjusting the voltage available at the output of said current source; a reference voltage source;

And means for injecting a Calibrating voltage in series with said reference voltage source and said precision resistor.

9. In a magnetometer:

A magnetic field detector including a sensitive element responsive to a component of the net magnetic field to which it is exposed in accordance with the strength of said component;

Winding means controlled by a current passing therethrough for producing a single variable neutralizing magnetic iield in opposition to an ambient magnetic iield to reduce the strength of the component to which said sensitive element is exposed; a series circuit including A precision resistor,

' A variable resistance means,

An adjustable eld balance resistor, said Winding means, and

A current source connected to supply a single current through said precision resistor, said variable resistance means, said iield balance resistor, Vand said winding means, the strength of said neutralizing eld being proportional to the strength of saidsingle current;

Electronic means for varying the magnitude of said variable resistance means as an inverse function of the magnitude of said component, whereby said single current varies in accordance with said said component;

A self-balancing recording potentiometer including a potentiometer resistance element having a movable contact and including a recording element movable with said contact to a point indicative of a voltage across an input circuit and also including means for driving a strip of recording medium past said Vrectlrlng' velement to ,remi ,Changes .in Said mirage;

A voltage source;

Means for connecting said precision resistor and said voltage source across the input circuit of said potentiometer;

And a source of reference voltage connected across said potentiometer resistance element.

10. In a magnetometer:

A magnetic eld detector including a sensitive element responsive tol a component of the net magnetic iield to which it is exposed-in accordance with the strength of said component;

Winding means controlled by a current passing therethrough for producing a single variable neutralizing magnetic ield in opposition to an ambient magnetic field to reduce the strength of the component to which said sensitive element is exposed;

A precision resistor;

A variable resistance means;

An adjustable eld balance resistor;

A current source connected to supply a single current through said precision -resistor, said variable resistance means, said lield balance resistor, and said 'winding means, the strength of said neutralizing ield being proportional to the strength of said single current; Y i

Means for varying the magnitude of said variable resistance means as an inverse function of the magnitude of said component, whereby said single current varies in accordance with said component;

A self-balancing recording potentiometer including a potentiometer resistance element having a movable contact and including a recording element movable with said contact to a point indicative of a voltage across an input circuit and also including means for driving a strip of recording medium past said recording element to record changes in said voltage;

A voltage source; Y

Means for connecting said precision resistorfand said .voltage source across the input circuit of said potentiometer; Y

' Sensitivity control resistance means;

Means for connecting said potentiometer resistance element and said sensitivity control resistance means across the output of said current source; a reference voltage source;

Means for injecting a Calibrating voltage in series with said reference voltage source and said precision resistor;

And means for adjusting the voltage available at the output of said clurrent source whereby the sensitivity of said potentiometer may be varied independently of the current flowing through said precision resistor.

ll. In a magnetometer:

A magnetic field detector including a sensitive element lresponsive to a component of the net magnetic eld to which it is exposed in accordance with the strength of said component;

Winding means controlled by a current passing therethrough for producing a single variable neutralizing magnetic lield in opposition to an ambient magnetic iield to reduce the strength of the component to which said sensitive element is exposed;

A single current source, two electron discharge devices connected in series with said source, said lsource continuously supplying a single direct current through said winding-means, the strength of said neutralizing eld being unidirectional and proportional to the amplitude of said single direct current; A

Means including a feed-back loop connected-to said winding means and one of said electron discharge devices for varying the magnitude of said single direct current in accordance with the magnitude of said compo-- nent;

And means for measuring said direct current to indicate the strength of the ambient magnetic ield. V- t l2. In a magnetometer: Y A magnetic eld detector including a sensitive element responsive to a component of the net magnetic iield to which it is exposed in accordance with the strength of said component;

Winding means controlled by a current passing therethrough for producing a single variable neutralizing magnetic eld in opposition to an ambient magnetic cld to reduce the strength of the component to which said sensitive element is exposed;

A variable resistance means,

A current source, two electron discharge devices connected in series With said source, said source continuously supplying a single direct current through said variable resistance means, and said winding means, the strength of said neutralizing eld being unidirectional and proportional to the strength of said single direct current;

Means including a feed-back loop connected to said winding means and one of said electron discharge devices for varying the magnitude of said variable resistance means as an inverse function of the magnitude of said component, whereby said single direct current varies in accordance with said component;

And means for measuring said direct current to indicate the strength of the ambient magnetic eld.

13. In a magnetometer:

A magnetic eld detector including a sensitive element responsive to a component of the net magnetic eld to which it is exposed in accordance with the strength of said component;

Winding means controlled by a current passing therethrough for producing a single variable neutralizing magnetic field in opposition to an ambient magnetic eld to reduce the strength of the component to which said sensitive element is exposed;

A variable resistance means;

An adjustable field balance resistor;

A current source, two electron discharge devices connected in series with said source, said source continuously supplying a single direct current through said variable resistor, said lield balance resistor, and said winding means, the strength of said neutralizing eld being unidirectional and proportional to the strength of said single direct current;

Means including a feed-back loop connected to said winding means and one of said electron discharge devices for varying the magnitude of said variable resistance means as an inverse function of the magnitude of said component, whereby said single direct current varies in accordance with said component;

And means for measuring said direct current to indicate the strength of the ambient magnetic field.

14. In a magnetometer,

A magnetic iield detecting means having an input and an output, said detecting means including an unbalanced pair of circuits linked to magnetic eld sensitive means responsive to a component of the net magnetic held to which the sensitive means is exposed;

An oscillator for applying an alternating electrical voltage to said input for saturating said magnetic eld sensitive means during alternate half-cycles of said alternating current voltage whereby alternate positive and negative pulses appear in said output, the magnitude of the positive pulses increasing and the magnitude of the negative pulses decreasing in accordance with the magnitude of said component of net magnetic eld, the frequency of occurrence of said positive and negative pulses being the same as the frequency of said alternating current voltage;

A circuit including a precision resistor, a variable resistance means, andl a direct current source, said circuit producing a unidirectional neutralizing magnetic iield having a strength proportional to the strength of the single direct current flowing through said resistor, said neutralizing magnetic eld being the only neutralizing eld and opposing the ambient magnetic field in the region of said magnetic eld sensitive means to reduce the net magnetic eld to which said sensitive means is exposed;

Instantaneously acting, solely electronic feedback means controlled by the output of said magnetic field detecting means and being instantaneously responsive to the diierence in magnitude of said alternate positive and negative pulses for varying the magnitude of said Variable resistance means as an inverse function of the magnitude of said component whereby said single direct current varies in accordance with the strength of said net magnetic eld; and

Means for measuring the voltage produced across said precision resistor to indicate the strength of said ambient magnetic eld.

15. A magnetometer as defined in claim 14 including an adjustable field balance resistor for adjusting the current flowing through said precision resistor.

16. A magnetometer as dened in claim 15 in which said precision resistor, variable resistance means, and direct current source are connected in series.

References Cited in the le of this patent UNITED STATES PATENTS 2,245,124 Bonn June 10, 1941 2,406,870 Vacquier Sept. 3, 1946 2,564,854 Muly Aug. 21, 1951 2,570,870 Schmitt et a1. Oct. 9, 1951 2,663,843 Wickerhan et al. Dec. 22, 1953 2,727,206 Ryerson Dec. 13, 1955 2,755,434 Yetter July 17, 1956 2,757,539 Broomell Aug. 7, 1956 

